The present invention relates to an alloy type thermal fuse, more particularly to improvement in an alloy type thermal fuse of an operating temperature of 57 to 67xc2x0 C., and also to a fuse element which constitutes such a fuse, and which is made of a low-melting fusible alloy.
In a conventional alloy type thermal fuse, a low-melting fusible alloy piece to which a flux is applied is used as a fuse element. Such a thermal fuse is mounted on an electric apparatus to be protected. When the electric apparatus abnormally generates heat, a phenomenon occurs in which the low-melting fusible alloy piece is liquefied by the generated heat, the molten metal is spheroidized by the surface tension under the coexistence with the flux that has already melted, and the alloy piece is finally broken as a result of advancement of the spheroidization, whereby the power supply to the apparatus is interrupted.
The first requirement which is imposed on such a low-melting fusible alloy is that the solid-liquid coexisting region between the solidus and liquidus lines is narrow. In an alloy, usually, a solid-liquid coexisting region exists between the solidus and liquidus lines. In this region, solid-phase particles are dispersed in a liquid phase, so that the region has also the property similar to that of a liquid phase, and therefore the above-mentioned breakage due to spheroidization may occur. As a result, there is the possibility that a low-melting fusible alloy piece is spheroidized and broken in a temperature range (indicated by xcex94T) which is lower than the liquidus temperature (indicated by T), and which belongs to the solid-liquid coexisting region. Therefore, a thermal fuse in which such a low-melting fusible alloy piece is used must be handled as a fuse which operates at a fuse element temperature in a range of (Txe2x88x92xcex94T) to T. As xcex94T is smaller, or as the solid-liquid coexisting region is narrower, the operating temperature of a thermal fuse is less dispersed, so that a thermal fuse can operate at a predetermined temperature in a correspondingly strict manner. Therefore, an alloy which is to be used as a fuse element of a thermal fuse is requested to have a narrow solid-liquid coexisting region.
The second requirement which is imposed on such a low-melting fusible alloy is that the electrical resistance is low. When the temperature rise by normal heat generation due to the resistance of the low-melting fusible alloy piece is indicated by xcex94Txe2x80x2, the operating temperature is substantially lower by xcex94Txe2x80x2 than that in the case where such a temperature rise does not occur. Namely, as xcex94Txe2x80x2 is larger, the operation error is substantially larger. Therefore, an alloy which is to be used as a fuse element of a thermal fuse is requested to have a low specific resistance.
A thermal fuse is repeatedly heated and cooled by heat cycles of an apparatus. During the heat cycles, re-crystalization of a fuse element is promoted. When the ductility of the fuse element is excessively large, larger distortion (slip) occurs in the interface between different phases in the alloy structure. When the distortion is repeated, a change in sectional area and an increase of the length of the fuse element are extremely caused. As a result, the resistance of the fuse element itself becomes unstable, and the thermal stability cannot be guaranteed. Therefore, also the thermal stability must be emphasized as a further requirement which is imposed on such a low-melting fusible alloy.
In order to severely manage an apparatus, recently, thermal fuses of an operating temperature of about 60xc2x0 C. are requested. In a fuse element of such a thermal fuse, it is necessary that the solid-liquid coexisting region is in the vicinity of 60xc2x0 C., and the above-mentioned xcex94T (the temperature range belonging to the solid-liquid coexisting region) must be within an allowable range (not larger than 4xc2x0 C.). As a low-melting fusible alloy of such a melting point, for example, known are, for example, an Inxe2x80x94Bixe2x80x94Cd alloy (61.7% In, 30.8% Bi, and 7.5% Cd (% means a weight percent (the same is applicable in the following description))) which is eutectic at 62xc2x0 C., an Inxe2x80x94Bixe2x80x94Sn alloy (51% In, 32.5% Bi, and 16.5% Sn) which is eutectic at 60xc2x0 C., and a Bixe2x80x94Inxe2x80x94Pbxe2x80x94Sn alloy (49% Bi, 21% In, 18% Pb, and 12% Sn) which is eutectic at 58xc2x0 C.
However, the Inxe2x80x94Bixe2x80x94Cd alloy which is eutectic at 62xc2x0 C. is not suitable to environment conservation which is a recent global request, because, among Pb, Cd, Hg, and Tl which are seemed to be harmful to the ecological system, Cd is contained in the alloy. In the alloy, In which is high in ductility occupies the majority of the composition, and hence the elastic limit is small. Therefore, the fuse element is caused to yield by thermal stress due to heat cycles, and a slip occurs in the alloy structure. As a result of repetition of such a slip, the sectional area and the length of the fuse element are changed, so that the resistance of the element itself is unstable and the thermal stability cannot be guaranteed.
The Bixe2x80x94Inxe2x80x94Pbxe2x80x94Sn alloy which is eutectic at 58xc2x0 C. is not suitable to environment conservation which is a recent global request, because Pb which is a metal harmful to the ecological system is contained in the alloy. The alloy contains a large amount of Bi, and therefore is so fragile that a process of drawing the alloy into a very thin wire of 300 xcexcmxcfx86 is hardly performed. Therefore, the alloy can hardly cope with the miniaturization of an alloy type thermal fuse which is conducted in accordance with the recent tendency that electric or electronic apparatuses are further reduced in size. In such a very thin fuse element, moreover, the relatively high specific resistance of the alloy composition cooperates with the thinness to extremely raise the resistance, with the result that an operation failure due to self-heating of the fuse element inevitably occurs.
In the Inxe2x80x94Bixe2x80x94Sn alloy which is eutectic at 60xc2x0 C., no harmful metal is contained, a process of drawing the alloy into a very thin wire of 300 xcexcmxcfx86 can be performed, and the specific resistance is low. In the same manner as the Inxe2x80x94Bixe2x80x94Cd alloy which is eutectic at 62xc2x0 C., however, In which is high in ductility occupies the majority of the composition, and hence the elastic limit is small. Therefore, the fuse element is caused to yield by thermal stress due to heat cycles, and a slip occurs in the alloy structure. As a result of repetition of such a slip, the sectional area and the length of the fuse element are changed, so that the resistance of the element itself is unstable and the thermal stability cannot be guaranteed.
It is an object of the invention to provide an alloy type thermal fuse in which an alloy composition of Inxe2x80x94Snxe2x80x94Bi is used as a fuse element, the operating temperature is in the range of 57 to 67xc2x0 C., requests for environment conservation can be satisfied, the diameter of the fuse element can be made very thin or reduced to about 300 xcexcmxcfx86, self-heating can be sufficiently suppressed, and the thermal stability can be satisfactorily guaranteed.
In one embodiment of the present invention, the alloy type thermal fuse is a thermal fuse in which a low-melting fusible alloy is used as a fuse element, wherein the low-melting fusible alloy has an alloy composition in which a total of 0.01 to 7 weight parts of at least one selected from the group consisting of Au, Ag, Cu, Ni, and Pd is added to 100 weight parts of a composition of 48 to 60% In, 10 to 25% Sn, and balance Bi.
In the above fuse, the alloy composition is allowed to contain inevitable impurities which are produced in productions of metals of raw materials and also in melting and stirring of the raw materials.